ng_device: replace dependency on vnode.h with fcntl.h

Update to current convention of using fcntl.h O_* flags instead of vnode.h IO_*
ones, as explained in sys/fs/devfs/devfs_vnops.c.

Signed-off-by: Quentin Thébault <quentin.thebault@defenso.fr>
Reviewed by: imp, jhb
Pull Request: https://github.com/freebsd/freebsd-src/pull/1827